Studies on the structure of coal have established that coal has a complex polymeric network containing ethers and short alkyl or alkylene chains as typical linking groups between substituted aromatic units typically with ring numbers of 1 to 4.
There are numerous processes for the conversion of coal to liquid hydrocarbon products involving hydroprocessing coal in the presence of a catalyst system. These processes typically utilize nickel, tin, molybdenum, cobalt, iron and vanadium containing catalysts, alone or in combination, with other metals such as selenium at high temperature alone or in combination with high hydrogen pressure. Coal can be impregnated with catalyst or the catalyst supported on a carrier. In some processes, coal is subjected to an initial solvent extraction prior to hydroprocessing. Solvents used for extraction include tetralin, decalin, alkyl substituted polycyclic aromatics, phenols and amines. Typical solvents are strong hydrogen donors.
Coal liquefaction may also be accomplished using combinations of catalysts with various solvents. Metal halides promoted with a mineral acid, ZnCl.sub.2 in the presence of-polar solvents and quinones in combination with ammonium ions, group 1a or 1b metal alkoxides or hydroxides or salts of weak acids have been used as catalyst systems for coal liquefaction. Aqueous solutions containing catalysts such as alkali metal silicates, calcium or magnesium ions and surfactants form media for breaking down coal.
Coal can be depolymerized into lower molecular weight fractions by breaking the ether, alkyl or alkylene bridging groups which collectively make up coal's polymeric structure. Catalysts for coal depolymerization include BF.sub.3 complexed with phenol, Bronsted acids such as H.sub.2 SO.sub.4, p-toluenesulfonic, trifluoromethanesulfonic and methanesulfonic acid in the presence of a phenolic solvent, ZnCl.sub.2 or FeCl.sub.3. This is followed by hydrotreatment. Depolymerization reactions have been reviewed by Wender et al, "Chemistry of Coal Utilization," 2nd Supplementary Volume, M. A. Elliot ed, J. Wiley & Sons, NY, 1981, pp. 425 et seq.
The high temperatures required by catalyzed coal liquefaction processes lead to refractory materials and liquefied hydrocarbon oils containing significant amounts of vacuum gas oil and other higher boiling components.
It is known in the art to use a hard acid soft base system (HSAB) to decompose coal. See for example, U.S. Pat. Nos. 5,298,157; 5,296,133 and 5,294,349. In such systems, coal is rapidly decomposed at low temperatures while minimizing the formation of refractory material by controlling side reactions leading to such materials. The finely divided coal particles are contacted with a hard acid in the presence of a soft base at temperatures of from 0.degree. C. to 100.degree. C., said hard acid being characterized by a heat of reaction with dimethylsulfide of from 10 KCal/mol to 30 KCal/mol and said soft base being characterized by a heat of reaction with boron trifluoride of from 10 KCal/mol to 17 KCal/mol. The decomposed coal can then be converted to low ash coal by extracting it to remove the hard acid and soft base. However, the extraction systems employed by the prior art have the questionable characteristic of removing many of the inorganic elements along with cleaved coal fragments resulting in sizable amounts of ash forming material as well as organic components being prematurely distributed to two locations, the extract and the residue.